Gender disparity has been recently reported for the severity of inflammatory bowel disease (IBD), with female patients with Crohn's disease (CD), one of the idiopathic IBDs, likely to experience more severe disease manifestations compared to males, and with symptom activity associated with hormonal surges during puberty, pregnancy and the post-partum period. Although the precise etiology is currently unknown, it is well accepted that IBD results from a dysregulated immune response to environmental factors (e.g., gut microflora) in genetically-predisposed individuals. However, the mechanism(s) underlying sex differences in IBD is an understudied area of investigation and little is known regarding how the orchestration of genetics, host immunity, as well as the gut microenvironment leads to gender disparity, specifically in Crohn's patients. Based on our extensive preliminary data, the central hypothesis of the present proposal is that gut microbial ligands signaling through TLR9 promote phenotypic and functional diversity in mucosal Treg populations that contribute to sex differences in experimental CD, such as that observed in SAMP mice, a spontaneous model of CD-like ileitis, and in patients with CD. This hypothesis will be tested by performing two specific aims: 1) Determine the impact of the gut microbiota on sex differences in the phenotypic and functional properties of the mucosal Treg population/subpopulations, and 2) Determine the mechanism(s) as to how TLR9 activation specifically results in sex-associated gut inflammation. The proposed studies are designed to elucidate potential mechanism(s) underlying sex differences in spontaneous, experimental Crohn's-like ileitis, and identify potential, early diseas pathways that can lead to the development of novel, gender-targeted strategies to improve treatment of patients suffering from this devastating disease.